This invention relates to a process for the production of fuel, for coal pressure gasification in a fixed bed reactor, from fine coal which is segregated by being graded into an oversize fraction and an undersize fraction, whereby the undersize fraction is pelleted and the pellets are hardened at temperatures below the coking temperature of coal and then subsequently added to the oversize fraction.
The production of special fuels for operating fixed bed reactors for coal pressure gasification aims, amongst other things, to minimise the dust content of the gasification gas and furthermore to achieve better draught in the fixed bed. The reduction of dust content of the gasification gas is essential in the gasification of coal, since the collection of dust in the gasification gas as a rule causes, amongst other things, a condensation of tar constituents and leads to a condensate which not only contains dust and tar but also produces an emulsion with the water content of the gas and the quench water which is hard to break. Under these circumstances thickening of the condensate leads to a viscous mass which is difficult to manipulate and handle. The reduction of the dust content therefore makes it possible to obtain highly fluid tar and hydrocarbons and a definite reduction in the difficulties which are otherwise connected with the handling of a thickened condensate.
It is known from German Auslegeschrift No. 108600 to apply a thickened condensate to the surface of the fixed bed, in order to moisten the whole area of the fixed bed with hydrocarbon compounds and to produce and maintain a type of oil filter in the fixed bed reactor with the desired filtering action and thickness for filtering off the dust. However, this known process does not have the desired result, since the necessary uniform distribution and increase in heat as far as the reaction area is not present in the fixed bed and also since the duration of the various fuel particles in the fixed bed shows great variation.
It is further known, from German Patent Specification No. 827224, to briquette wash waste and flotation waste from coal preparation without a binding agent and to gasify the briquettes into a green state, since such briquettes have sufficient heat resistance. This process cannot be used for the gasification of fine coal, since briquetting without a binding agent results only in the formation of carbon granules and not in the binding together of the carbon granules. The gasification of fine coal is, however, essential for coal pressure gasification in gas/steam turbine power plants, since around 70% of the coal in the usual process for obtaining and preparing pit coal is produced as fine coal and the proportion of undersize granules of 0-2 mm conclusively determines the dust content in the gasifying gas.
In the prior art there is also a process in which the undersize fraction of 0-2 mm which is separated from the oversize fraction of 2-3 mm is pelleted by means of a thickened condensate and the green pellets are added to the oversize fraction (German Offenlegungsschrift No. 2,540,165). However it has proved that these green pellets do not have the necessary fire resistance. In any case, a large proportion of the pellets do not stay firmly in the fixed bed since the lighter parts of the binding agent vaporise too quickly and the pellets do not reach the reaction area. Moreover, the pellets are not abrasion-proof so that considerable amounts of dust are released, particularly in the transport conduits.
Finally, the process described at the outset is known from U.S. Pat. No. 3,692,505. This covers in particular the production of fuel from bituminous coal which has a substantially reduced coking capacity and therefore eliminates the difficulties which arise in the fixed bed with bituminous coal. With this process pelleting is achieved with hot semicoke particles which form nuclei around which the fine grain particles of the pelleted material are arranged. By maintaining the temperatures in the semicoke particles at an exact level and temperatures in the pelleting drum between 330.degree. C. and 600.degree. C., the pellets sinter by agglomeration of the fine coal particles and develop no coking effect in the fixed bed.
This process has the disadvantage that, outside the fixed bed reactor, a considerable quantity of gas is formed from the coal in the production of semicoke particles and in the sintering of the pellets, which is lost for the gasifying gas. Moreover, the known process presumes certain types of coal for the production of the fuel and therefore does not adequately exclude bituminous coal, which nevertheless causes considerable amounts of dust in the gasifying gas.
The invention is based on the problem of producing fuel pellets having better fire resistance and less sensitivity to abrasion and providing them in the fuel, whilst maintaining the coal in the original state until the fuel is fed into the reactor, so as to prevent losses in the gasification gas.